A spark-ignited internal-combustion engine generally operates by combusting a fuel in the presence of an oxidizer such as air. If the internal-combustion engine is to operate efficiently, the equivalence ratio must be effectively controlled. The equivalence ratio is a term used to collectively represent the fuel to air ratio, which is denoted by the Greek letter phi (φ), and the air to fuel ratio, which is denoted by Greek letter lambda (λ) and is the inverse of phi.
Some engine systems employ a trim system to accomplish the task of controlling the equivalence ratio. The trim system operates in conjunction with a fuel system and an equivalence ratio feedback system to maintain a desirable equivalence ratio. Preferably for systems utilizing a 3-way catalyst, the trim system in conjunction with the feedback system is able to keep the equivalence ratio at or near stoichiometry. Stoichiometry is the point at which the most complete combustion takes place in the internal-combustion engine. Operating at stoichoimetric conditions results in the highest three-way catalyst efficiency which equates to the lowest overall emissions out of the catalyst.
While many trim systems are known in the art, many existing systems have problems. For example, some trim systems produce undesirable pressure pulsations during operation. In extreme cases, the pressure pulsations cause noticeable variations in the equivalence ratio. Unintended variations in the equivalence ratio also increase emissions. The pressure pulsations can negatively affect components in the fuel system. The pressure pulsations can excite a converter (i.e., regulator, vaporizer) in the fuel system such that internal components of the converter are subject to premature wear. As a result, the converter and other components have to be replaced more often than expected. Replacing worn components is time consuming, costly and may not be allowed without penalties from governing bodies.
Additionally, some trim systems employ only a single solenoid trim valve to regulate the equivalence ratio. In such systems, there is no redundancy built into the trim system. If the lone solenoid trim valve fails, the trim system is no longer able to control the equivalence ratio. As a result, the performance of the internal-combustion engine suffers dramatically.
Further, known trim systems are not designed to handle a loss of trim system power and still maintain the equivalence ratio at or near stoichiometry. Should a loss of power occur, the fuel to air mixture is uncontrolled and, in most cases, becomes either very rich or very lean. The end result is that the internal-combustion engine operates poorly and emissions from the engine are increased when trim system power is lost.
From the foregoing, it can be seen that existing trim systems have problems associated with them.